A gas turbine engine effective for powering an aircraft at supersonic flight typically includes a variable area exhaust nozzle having primary and secondary exhaust flaps which are joined together to define a throat of minimum flow area with a converging channel defined by the primary flaps and a diverging flow channel defined by the secondary flaps. The primary and secondary flaps are conventionally movable between a generally closed position having a minimum throat area and a minimum outlet area at the aft end of the secondary flaps, and to a generally fully open position having a maximum throat area and a maximum outlet area.
Such converging-diverging (C-D) exhaust nozzles may either be axisymmetric or two-dimensional, or rectangular. In a two-dimensional C-D nozzle, the primary and secondary flaps extend transversely between a pair of spaced, nozzle sidewalls which collectively bound the exhaust gases being channeled therethrough. Since the primary and secondary flaps move relative to the stationary sidewalls, suitable seals are typically provided therebetween for reducing, if not eliminating, leakage of the exhaust gases from the converging and diverging channels, between the flaps and the sidewall, and outwardly therefrom. Leakage of the hot exhaust gases would require suitable thermal protection of components adjacent to the exhaust nozzle including, for example, hydraulic and electrical systems. Leakage of the exhaust gases also may decrease the propulsion efficiency of the exhaust nozzle.
During operation of the engine and exhaust nozzle, the nozzle sidewall is subject to distortion due to high temperatures thereof, or pressure forces from the exhaust gases, or both. Such distortion may be in the form of a bow which may occur in the transverse direction of the exhaust nozzle generally parallel to the exhaust flaps and the longitudinal centerline axis. A conventional exhaust flap seal at the nozzle sidewall typically includes a straight seal which would contact the bowed sidewall at only two points leaving an arcuate gap between the bowed sidewall and the seal through which the exhaust gases will leak.
Furthermore, in one embodiment or a two-dimensional exhaust nozzle, the secondary exhaust flaps are disposed fully between the adjacent sidewalls when in the closed position. However, when the secondary flaps are disposed in the open position, the downstream end portions thereof extend freely over edges of the nozzle sidewalls. In this latter condition, portions of the nozzle seals will also extend over the edges of the nozzle sidewall. Accordingly, the nozzle seal must be suitably joined to the secondary flap to ensure that it does not separate from the flap in the open position, and that it smoothly slides back against the nozzle sidewall as the secondary flap is moved toward the closed position.